Karl Popper

Karl Popper is generally regarded as one of the greatest philosophers
of science of the 20th century. He was also a social and political
philosopher of considerable stature, a self-professed
critical-rationalist, a dedicated opponent of all forms of scepticism,
conventionalism, and relativism in science and in human affairs
generally and a committed advocate and staunch defender of the
‘Open Society’. One of the many remarkable features of
Popper’s thought is the scope of his intellectual influence: he
was lauded by Bertrand Russell, taught Imre Lakatos, Paul Feyerabend
and the future billionaire investor and philanthropist George Soros at
the London School of Economics, numbered David Miller, Joseph Agassi,
Alan Musgrave and Jeremy Shearmur amongst his research assistants
there and had reciprocally beneficial friendships with the economist
Friedrich Hayek and the art historian Ernst Gombrich. Additionally,
Peter Medawar, John Eccles and Hermann Bondi are amongst the
distinguished scientists who have acknowledged their intellectual
indebtedness to his work, the latter declaring that “There is no
more to science than its method, and there is no more to its method
than Popper has said.”

1. Life

Karl Raimund Popper was born on 28 July 1902 in Vienna, which at that
time could make some claim to be the cultural epicentre of the western
world. His parents, who were of Jewish origin, brought him up in an
atmosphere which he was later to describe as ‘decidedly
bookish’. His father was a lawyer by profession, but he also
took a keen interest in the classics and in philosophy, and
communicated to his son an interest in social and political issues
which he was to never lose. His mother inculcated in him such a
passion for music that for a time he seriously contemplated taking it
up as a career, and indeed he initially chose the history of music as
a second subject for his Ph.D. examination. Subsequently, his love for
music became one of the inspirational forces in the development of his
thought, and manifested itself in his highly original interpretation
of the relationship between dogmatic and critical thinking, in his
account of the distinction between objectivity and subjectivity, and,
most importantly, in the growth of his hostility towards all forms of
historicism, including historicist ideas about the nature of the
‘progressive’ in music. The young Karl attended the local
Realgymnasium, where he was unhappy with the standards of the
teaching, and, after an illness which kept him at home for a number of
months, he left to attend the University of Vienna in 1918. However,
he did not formally enroll at the University by taking the
matriculation examination for another four years. 1919 was in many
respects the most important formative year of his intellectual life.
In that year he became heavily involved in left-wing politics, joined
the Association of Socialist School Students, and became for a time a
Marxist. However, he was quickly disillusioned with the doctrinaire
character of the latter, and soon abandoned it entirely. He also
discovered the psychoanalytic theories of Freud and Adler (he served
briefly as a voluntary social worker with deprived children in one of
the latter’s clinics in the 1920s), and listened entranced to a
lecture which Einstein gave in Vienna on relativity theory. The
dominance of the critical spirit in Einstein, and its total absence in
Marx, Freud and Adler, struck Popper as being of fundamental
importance: the pioneers of psychoanalysis, he came to think, couched
their theories in terms which made them amenable only to confirmation,
while Einstein’s theory, crucially, had testable implications
which, if false, would have falsified the theory itself.

Popper had a rather melancholic personality and took some time to
settle on a career; he trained as a cabinetmaker, obtained a primary
school teaching diploma in 1925 and qualified to teach mathematics and
physics in secondary school in 1929. He undertook a doctoral programme
with the department of psychology at the University of Vienna the
under the supervision of Karl Bühler, who, with Otto Külpe, was one
of the founder members of the Würzburg school of experimental
psychology. Popper’s project was initially designed as a
psychological investigation of human memory, on which he had conducted
initial research. However, the subject matter of a planned
introductory chapter on methodology assumed a position of increasing
pre-eminence and this resonated with Bühler, who, as a distinguished
Kantian scholar, a professor of philosophy as well as psychology, had
famously addressed the issue of the contemporary ‘crisis in
psychology’. This ‘crisis’, for Bühler, related to the question
of the unity of psychology and had been generated by the proliferation
of then competing paradigms within psychology which had undermined the
hitherto dominant associationist one and problematized the question of
method. Accordingly, under Bühler’s direction, Popper switched his
topic to the methodological problem of cognitive psychology and
received his doctorate in 1928 for his dissertation “Die Methodenfrage der
Denkpsychologie”. In extending Bühler’s Kantian approach to
the crisis in the dissertation, Popper critiqued Moritz Schlick’s
physicalist programme for a scientific psychology based ultimately
upon the transformation of psychology into a science of brain
processes. This latter ideal, Popper argued, was misconceived, but the
issues raised by it ultimately had the effect of refocusing his
attention from Bühler’s question of the unity of psychology to that
of its scientificity, and this philosophical focus on questions of
method, objectivity and claims to scientific status was to become a
principal life-long concern for him. This also brought the orientation
of his thought into line with that of such contemporary ‘analytic’
philosophers as Frege and Russell as well as that of many members of
the Vienna Circle and led him to effectively abandon psychology for
philosophy of science.

Popper married Josephine Anna Henninger (‘Hennie’) in
1930, and she oversaw his welfare with unflagging support and
devotion, serving additionally as his amanuensis until her death in
1985. At an early stage of their marriage they decided that they would
never have children, a decision which Popper was able to look back on
in later life with apparent equanimity. In 1937 Popper took up a
position teaching philosophy at the University of Canterbury in New
Zealand, where he was to remain for the duration of the Second World
War, though he had a rather tense relationship with his head of
department. Additionally, Hennie had difficulty adapting to life away
from her native Vienna and homesickness made her increasingly unhappy;
this was exacerbated by the sheer relentlessness of Popper’s personal
work ethic, which they both found exhausting.

The annexation of Austria in 1938 became the catalyst which prompted
Popper to refocus his writings on social and political philosophy and
he published The Open Society and Its Enemies, his critique
of totalitarianism, in 1945. In 1946 he moved to England to teach at
the London School of Economics, and became professor of logic and
scientific method at the University of London in 1949. From this point
on his reputation and stature as a philosopher of science and social
thinker grew enormously, and he continued to write prolifically—a
number of his works, particularly The Logic of Scientific
Discovery (1959), are now widely seen as pioneering classics in
the field. However, he combined a combative personality with a zeal
for self-aggrandisement that did little to endear him to professional
colleagues at a personal level. He was ill-at-ease in the
philosophical milieu of post-war Britain which was, as he saw it,
fixated with trivial linguistic concerns dictated by Wittgenstein,
whom he considered to be his nemesis. Popper was a somewhat
paradoxical man, whose theoretic commitment to the primacy of rational
criticism was counterpointed by hostility towards anything that
amounted to less than total acceptance of his own thought, and in
Britain—as had been the case in Vienna—he became
increasingly an isolated figure, though his ideas continued to inspire
admiration.

In later years Popper came under philosophical criticism for his
prescriptive approach to science and his emphasis on the logic of
falsification. This was superseded in the eyes of many by the
socio-historical approach taken by Thomas Kuhn in The Structure of
Scientific Revolutions (1962), who—in arguing for the
incommensurability of rival scientific paradigms—reintroduced
the idea that change in science is essentially dialectical and is
dependent upon the establishment of consensus within communities of
researchers.

Popper was knighted in 1965, and retired from the University of London
in 1969, though he remained active as a writer, broadcaster and
lecturer until his death in 1994. (For more detail on Popper’s life,
cf. his Unended Quest).

2. Backdrop to his Thought

A number of biographical features may be identified as having a
particular influence upon Popper’s thought. In the first place, his
teenage flirtation with Marxism left him thoroughly familiar with the
Marxist view of economics, class-war, and history. Secondly, he was
appalled by the failure of the democratic parties to stem the rising
tide of fascism in his native Austria in the 1920s and 1930s, and the
effective welcome extended to it by the Marxists. The latter acted on
the ideological grounds that it constituted what they believed to be a
necessary dialectical step towards the implosion of capitalism and the
ultimate revolutionary victory of communism. This was one factor which
led to the much feared Anschluss, the annexation of Austria
by the German Reich, the anticipation of which forced Popper into
permanent exile from his native country. The Poverty of
Historicism (1944) and The Open Society and Its Enemies
(1945), his most impassioned and brilliant social works, are as a
consequence a powerful defence of democratic liberalism as a social
and political philosophy, and a devastating critique of the principal
philosophical presuppositions underpinning all forms of
totalitarianism. Thirdly, as we have seen, Popper was profoundly
impressed by the differences between the allegedly
‘scientific’ theories of Freud and Adler and the
revolution effected by Einstein’s theory of relativity in physics in
the first two decades of this century. The main difference between
them, as Popper saw it, was that while Einstein’s theory was highly
‘risky’, in the sense that it was possible to deduce
consequences from it which were, in the light of the then dominant
Newtonian physics, highly improbable (e.g., that light is deflected
towards solid bodies—confirmed by Eddington’s experiments in
1919), and which would, if they turned out to be false, falsify the
whole theory, nothing could, even in principle, falsify
psychoanalytic theories. These latter, Popper came to feel, have more
in common with primitive myths than with genuine science. That is to
say, he saw that what is apparently the chief source of strength of
psychoanalysis, and the principal basis on which its claim to
scientific status is grounded, viz. its capability to accommodate, and
explain, every possible form of human behaviour, is in fact a critical
weakness, for it entails that it is not, and could not be, genuinely
predictive. Psychoanalytic theories by their nature are insufficiently
precise to have negative implications, and so are immunised from
experiential falsification.

The Marxist account of history too, Popper held, is not scientific,
although it differs in certain crucial respects from psychoanalysis.
For Marxism, Popper believed, had been initially scientific, in that
Marx had postulated a theory which was genuinely predictive. However,
when these predictions were not in fact borne out, the theory was
saved from falsification by the addition of ad hoc hypotheses
which made it compatible with the facts. By this means, Popper
asserted, a theory which was initially genuinely scientific
degenerated into pseudo-scientific dogma.

These factors combined to make Popper take falsifiability as
his criterion for demarcating science from non-science: if a theory is
incompatible with possible empirical observations it is scientific;
conversely, a theory which is compatible with all such observations,
either because, as in the case of Marxism, it has been modified solely
to accommodate such observations, or because, as in the case of
psychoanalytic theories, it is consistent with all possible
observations, is unscientific. For Popper, however, to assert that a
theory is unscientific, is not necessarily to hold that it is
unenlightening, still less that it is meaningless, for it sometimes
happens that a theory which is unscientific (because it is
unfalsifiable) at a given time may become falsifiable, and thus
scientific, with the development of technology, or with the further
articulation and refinement of the theory. Further, even purely
mythogenic explanations have performed a valuable function in the past
in expediting our understanding of the nature of reality.

3. The Problem of Demarcation

As Popper represents it, the central problem in the philosophy of
science is that of demarcation, i.e., of distinguishing between
science and what he terms ‘non-science’, under which
heading he ranks, amongst others, logic, metaphysics, psychoanalysis,
and Adler’s individual psychology. Popper is unusual amongst
contemporary philosophers in that he accepts the validity of
the Humean critique of induction, and indeed, goes beyond it in
arguing that induction is never actually used in science. However, he
does not concede that this entails the scepticism which is associated
with Hume, and argues that the Baconian/Newtonian insistence on the
primacy of ‘pure’ observation, as the initial step in the
formation of theories, is completely misguided: all observation is
selective and theory-laden—there are no pure or theory-free
observations. In this way he destabilises the traditional view that
science can be distinguished from non-science on the basis of its
inductive methodology; in contradistinction to this, Popper holds that
there is no unique methodology specific to science. Science, like
virtually every other human, and indeed organic, activity, Popper
believes, consists largely of problem-solving.

Popper accordingly repudiates induction and rejects the view that it
is the characteristic method of scientific investigation and
inference, substituting falsifiability in its place. It is
easy, he argues, to obtain evidence in favour of virtually any theory,
and he consequently holds that such ‘corroboration’, as he
terms it, should count scientifically only if it is the positive
result of a genuinely ‘risky’ prediction, which might
conceivably have been false. For Popper, a theory is scientific only
if it is refutable by a conceivable event. Every genuine test of a
scientific theory, then, is logically an attempt to refute or to
falsify it, and one genuine counter-instance falsifies the whole
theory. In a critical sense, Popper’s theory of demarcation is based
upon his perception of the logical asymmetry which holds between
verification and falsification: it is logically impossible to
conclusively verify a universal proposition by reference to experience
(as Hume saw clearly), but a single counter-instance conclusively
falsifies the corresponding universal law. In a word, an exception,
far from ‘proving’ a rule, conclusively refutes it.

Every genuine scientific theory then, in Popper’s view, is
prohibitive, in the sense that it forbids, by implication,
particular events or occurrences. As such it can be tested and
falsified, but never logically verified. Thus Popper stresses that it
should not be inferred from the fact that a theory has withstood the
most rigorous testing, for however long a period of time, that it has
been verified; rather we should recognise that such a theory has
received a high measure of corroboration. and may be provisionally
retained as the best available theory until it is finally falsified
(if indeed it is ever falsified), and/or is superseded by a better
theory.

Popper has always drawn a clear distinction between the
logic of falsifiability and its applied methodology. The
logic of his theory is utterly simple: if a single ferrous metal is
unaffected by a magnetic field it cannot be the case that all ferrous
metals are affected by magnetic fields. Logically speaking, a
scientific law is conclusively falsifiable although it is not
conclusively verifiable. Methodologically, however, the situation is
much more complex: no observation is free from the possibility of
error—consequently we may question whether our experimental
result was what it appeared to be.

Thus, while advocating falsifiability as the criterion of demarcation
for science, Popper explicitly allows for the fact that in practice a
single conflicting or counter-instance is never sufficient
methodologically to falsify a theory, and that scientific theories are
often retained even though much of the available evidence conflicts
with them, or is anomalous with respect to them. Scientific theories
may, and do, arise genetically in many different ways, and the manner
in which a particular scientist comes to formulate a particular theory
may be of biographical interest, but it is of no consequence as far as
the philosophy of science is concerned. Popper stresses in particular
that there is no unique way, no single method such as induction, which
functions as the route to scientific theory, a view which Einstein
personally endorsed with his affirmation that ‘There is no
logical path leading to [the highly universal laws of science]. They
can only be reached by intuition, based upon something like an
intellectual love of the objects of experience’. Science, in
Popper’s view, starts with problems rather than with
observations—it is, indeed, precisely in the context of
grappling with a problem that the scientist makes observations in the
first instance: his observations are selectively designed to test the
extent to which a given theory functions as a satisfactory solution to
a given problem.

On this criterion of demarcation physics, chemistry, and
(non-introspective) psychology, amongst others, are sciences,
psychoanalysis is a pre-science (i.e., it undoubtedly contains useful
and informative truths, but until such time as psychoanalytical
theories can be formulated in such a manner as to be falsifiable, they
will not attain the status of scientific theories), and astrology and
phrenology are pseudo-sciences. Formally, then, Popper’s theory of
demarcation may be articulated as follows: where a ‘basic
statement’ is to be understood as a particular
observation-report, then we may say that a theory is scientific if and
only if it divides the class of basic statements into the following
two non-empty sub-classes: (a) the class of all those basic statements
with which it is inconsistent, or which it prohibits—this is the
class of its potential falsifiers (i.e., those statements
which, if true, falsify the whole theory), and (b) the class of those
basic statements with which it is consistent, or which it permits
(i.e., those statements which, if true, corroborate it, or bear it
out).

4. The Growth of Human Knowledge

For Popper accordingly, the growth of human knowledge proceeds from
our problems and from our attempts to solve them. These attempts
involve the formulation of theories which, if they are to explain
anomalies which exist with respect to earlier theories, must go beyond
existing knowledge and therefore require a leap of the imagination.
For this reason, Popper places special emphasis on the role played by
the independent creative imagination in the formulation of theory. The
centrality and priority of problems in Popper’s account of
science is paramount, and it is this which leads him to characterise
scientists as ‘problem-solvers’. Further, since the
scientist begins with problems rather than with observations or
‘bare facts’, Popper argues that the only logical
technique which is an integral part of scientific method is that of
the deductive testing of theories which are not themselves the product
of any logical operation. In this deductive procedure conclusions are
inferred from a tentative hypothesis. These conclusions are then
compared with one another and with other relevant statements to
determine whether they falsify or corroborate the hypothesis. Such
conclusions are not directly compared with the facts, Popper stresses,
simply because there are no ‘pure’ facts available; all
observation-statements are theory-laden, and are as much a function of
purely subjective factors (interests, expectations, wishes, etc.) as
they are a function of what is objectively real.

How then does the deductive procedure work? Popper specifies four
steps (Logic of Scientific Discovery, 1.3, 9):

(a) The first is formal, a testing of the internal
consistency of the theoretical system to see if it involves any
contradictions.

(b) The second step is semi-formal, the axiomatising of the
theory to distinguish between its empirical and its logical elements.
In performing this step the scientist makes the logical form of the
theory explicit. Failure to do this can lead to
category-mistakes—the scientist ends up asking the wrong
questions, and searches for empirical data where none are available.
Most scientific theories contain analytic (i.e., a priori)
and synthetic elements, and it is necessary to axiomatise them in
order to distinguish the two clearly.

(c) The third step is the comparing of the new theory with existing
ones to determine whether it constitutes an advance upon them. If it
does not constitute such an advance, it will not be adopted. If, on
the other hand, its explanatory success matches that of the existing
theories, and additionally, it explains some hitherto anomalous
phenomenon, or solves some hitherto unsolvable problems, it will be
deemed to constitute an advance upon the existing theories, and will
be adopted. Thus science involves theoretical progress. However,
Popper stresses that we ascertain whether one theory is better than
another by deductively testing both theories, rather than by
induction. For this reason, he argues that a theory is deemed to be
better than another if (while unfalsified) it has greater empirical
content, and therefore greater predictive power than its rival. The
classic illustration of this in physics was the replacement of
Newton’s theory of universal gravitation by Einstein’s theory of
relativity. This elucidates the nature of science as Popper sees it:
at any given time there will be a number of conflicting theories or
conjectures, some of which will explain more than others. The latter
will consequently be provisionally adopted. In short, for Popper any
theory \(X\) is better than a ‘rival’ theory
\(Y\) if \(X\) has greater empirical content, and
hence greater predictive power, than \(Y\).

(d) The fourth and final step is the testing of a theory by the
empirical application of the conclusions derived from it. If such
conclusions are shown to be true, the theory is corroborated (but
never verified). If the conclusion is shown to be false, then this is
taken as a signal that the theory cannot be completely correct
(logically the theory is falsified), and the scientist begins his
quest for a better theory. He does not, however, abandon the
present theory until such time as he has a better one to substitute
for it. More precisely, the method of theory-testing is as follows:
certain singular propositions are deduced from the new
theory—these are predictions, and of special interest are those
predictions which are ‘risky’ (in the sense of being
intuitively implausible or of being startlingly novel) and
experimentally testable. From amongst the latter the scientist next
selects those which are not derivable from the current or existing
theory—of particular importance are those which contradict the
current or existing theory. He then seeks a decision as regards these
and other derived statements by comparing them with the results of
practical applications and experimentation. If the new predictions are
borne out, then the new theory is corroborated (and the old
one falsified), and is adopted as a working hypothesis. If the
predictions are not borne out, then they falsify the theory from which
they are derived. Thus Popper retains an element of empiricism: for
him scientific method does involve making an appeal to experience. But
unlike traditional empiricists, Popper holds that experience cannot
determine theory (i.e., we do not argue or infer from
observation to theory), it rather delimits it: it shows which
theories are false, not which theories are true. Moreover, Popper also
rejects the empiricist doctrine that empirical observations are, or
can be, infallible, in view of the fact that they are themselves
theory-laden.

The general picture of Popper’s philosophy of science, then is this:
Hume’s philosophy demonstrates that there is a contradiction implicit
in traditional empiricism, which holds both that all knowledge is
derived from experience and that universal propositions
(including scientific laws) are verifiable by reference to experience.
The contradiction, which Hume himself saw clearly, derives from the
attempt to show that, notwithstanding the open-ended nature of
experience, scientific laws may be construed as empirical
generalisations which are in some way finally confirmable by a
‘positive’ experience. Popper eliminates the contradiction
by rejecting the first of these principles and removing the demand for
empirical verification in favour of empirical falsification in the
second. Scientific theories, for him, are not inductively inferred
from experience, nor is scientific experimentation carried out with a
view to verifying or finally establishing the truth of theories;
rather, all knowledge is provisional, conjectural,
hypothetical—we can never finally prove our scientific
theories, we can merely (provisionally) confirm or (conclusively)
refute them; hence at any given time we have to choose between the
potentially infinite number of theories which will explain the set of
phenomena under investigation. Faced with this choice, we can only
eliminate those theories which are demonstrably false, and rationally
choose between the remaining, unfalsified theories. Hence Popper’s
emphasis on the importance of the critical spirit to science—for
him critical thinking is the very essence of rationality. For it is
only by critical thought that we can eliminate false theories, and
determine which of the remaining theories is the best available one,
in the sense of possessing the highest level of explanatory force and
predictive power. It is precisely this kind of critical thinking which
is conspicuous by its absence in contemporary Marxism and in
psychoanalysis.

5. Probability, Knowledge and Verisimilitude

In the view of many social scientists, the more probable a theory is,
the better it is, and if we have to choose between two
theories which are equally strong in terms of their explanatory power,
and differ only in that one is probable and the other is improbable,
then we should choose the former. Popper rejects this. Science, or to
be precise, the working scientist, is interested, in Popper’s view, in
theories with a high informative content, because such theories
possess a high predictive power and are consequently highly testable.
But if this is true, Popper argues, then, paradoxical as it may sound,
the more improbable a theory is the better it is
scientifically, because the probability and informative content of a
theory vary inversely—the higher the informative content of a
theory the lower will be its probability, for the more information a
statement contains, the greater will be the number of ways in which it
may turn out to be false. Thus the statements which are of special
interest to the scientist are those with a high informative content
and (consequentially) a low probability, which nevertheless come close
to the truth. Informative content, which is in inverse proportion to
probability, is in direct proportion to testability. Consequently the
severity of the test to which a theory can be subjected, and by means
of which it is falsified or corroborated, is all-important.

For Popper, all scientific criticism must be piecemeal, i.e., he holds
that it is not possible to question every aspect of a theory at once.
More precisely, while attempting to resolve a particular problem a
scientist of necessity accepts all kinds of things as unproblematic.
These things constitute what Popper terms the ‘background
knowledge’. However, he stresses that the background knowledge
is not knowledge in the sense of being conclusively
established; it may be challenged at any time, especially if it is
suspected that its uncritical acceptance may be responsible for
difficulties which are subsequently encountered. Nevertheless, it is
clearly not possible to question both the theory and the background
knowledge at the same time (e.g., in conducting an experiment the
scientist of necessity assumes that the apparatus used is in working
order).

How then can one be certain that one is questioning the right thing?
The Popperian answer is that we cannot have absolute certainty here,
but repeated tests usually show where the trouble lies. Even
observation statements, Popper maintains, are fallible, and science in
his view is not a quest for certain knowledge, but an evolutionary
process in which hypotheses or conjectures are imaginatively proposed
and tested in order to explain facts or to solve problems. Popper
emphasises both the importance of questioning the background knowledge
when the need arises, and the significance of the fact that
observation-statements are theory-laden, and hence fallible. For while
falsifiability is simple as a logical principle, in practice it is
exceedingly complicated—no single observation can ever be taken
to falsify a theory, for there is always the possibility (a) that the
observation itself is mistaken, or (b) that the assumed background
knowledge is faulty or defective.

Popper was initially uneasy with the concept of truth, and in his
earliest writings he avoided asserting that a theory which is
corroborated is true—for clearly if every theory is an
open-ended hypothesis, as he maintains, then ipso facto it
has to be at least potentially false. For this reason Popper
restricted himself to the contention that a theory which is falsified
is false and is known to be such, and that a theory which replaces a
falsified theory (because it has a higher empirical content than the
latter, and explains what has falsified it) is a ‘better
theory’ than its predecessor. However, he came to accept
Tarski’s reformulation of the correspondence theory of truth, and in
Conjectures and Refutations (1963) he integrated the concepts
of truth and content to frame the metalogical concept of
‘truthlikeness’ or ‘verisimilitude’.
A ‘good’ scientific theory, Popper thus argued, has a
higher level of verisimilitude than its rivals, and he explicated this
concept by reference to the logical consequences of theories. A
theory’s content is the totality of its logical consequences, which
can be divided into two classes: there is the
‘truth-content’ of a theory, which is the class
of true propositions which may be derived from it, on the one hand,
and the ‘falsity-content’ of a theory, on the
other hand, which is the class of the theory’s false consequences
(this latter class may of course be empty, and in the case of a theory
which is true is necessarily empty).

Popper offered two methods of comparing theories in terms of
verisimilitude, the qualitative and quantitative definitions. On the
qualitative account, Popper asserted:

Assuming that the truth-content and the falsity-content of two
theories \(t_1\) and \(t_2\) are
comparable, we can say that \(t_2\) is more closely
similar to the truth, or corresponds better to the facts, than
\(t_1\), if and only if either:

(a) the truth-content but not the falsity-content of
\(t_2\) exceeds that of \(t_1\), or

(b) the falsity-content of \(t_1\), but not its
truth-content, exceeds that of \(t_2\).
(Conjectures and Refutations, 233).

Here, verisimilitude is defined in terms of subclass relationships:
\(t_2\) has a higher level of verisimilitude than
\(t_1\) if and only if their truth- and
falsity-contents are comparable through subclass relationships, and
either (a) \(t_2\)’s truth-content includes
\(t_1\)’s and \(t_2\)’s falsity-content,
if it exists, is included in, or is the same as,
\(t_1\)’s, \(or\) (b) \(t_2\)’s
truth-content includes or is the same as \(t_1\)’s and
\(t_2\)’s falsity-content, if it exists, is included in
\(t_1\)’s.

On the quantitative account, verisimilitude is defined by assigning
quantities to contents, where the index of the content of a given
theory is its logical improbability (given again that content and
probability vary inversely). Formally, then, Popper defines the
quantitative verisimilitude which a statement ‘a’
possesses by means of a formula:

\[
Vs(a) = Ct_T (a) - Ct_F (a),
\]

where \(Vs(a)\) represents the verisimilitude of \(a\), \(Ct_T (a)\)
is a measure of the truth-content of \(a\), and \(Ct_F (a)\) is a
measure of its falsity-content.

The utilisation of either method of computing verisimilitude shows,
Popper held, that even if a theory \(t_2\) with a
higher content than a rival theory \(t_1\) is
subsequently falsified, it can still legitimately be regarded as a
better theory than \(t_1\), and ‘better’ is
here now understood to mean \(t_2\) is closer to
the truth than \(t_1\). Thus scientific progress
involves, on this view, the abandonment of partially true, but
falsified, theories, for theories with a higher level of
verisimilitude, i.e., which approach more closely to the truth. In
this way, verisimilitude allowed Popper to mitigate what many saw as
the pessimism of an anti-inductivist philosophy of science which held
that most, if not all scientific theories are false, and that a true
theory, even if discovered, could not be known to be such.
With the introduction of the new concept, Popper was able to represent
this as an essentially optimistic position in terms of which we can
legitimately be said to have reason to believe that science makes
progress towards the truth through the falsification and corroboration
of theories. Scientific progress, in other words, could now be
represented as progress towards the truth, and experimental
corroboration could be seen an indicator of
verisimilitude.

However, in the 1970s a series of papers published by researchers
such as Miller, Tichý, and Grünbaum in particular revealed
fundamental defects in Popper’s formal definitions of verisimilitude.
The significance of this work was that verisimilitude is largely
important in Popper’s system because of its application to theories
which are known to be false. In this connection, Popper had
written:

Ultimately, the idea of verisimilitude is most important in cases
where we know that we have to work with theories which are at
best approximations—that is to say, theories of which we
know that they cannot be true. (This is often the case in the social
sciences). In these cases we can still speak of better or worse
approximations to the truth (and we therefore do not need to interpret
these cases in an instrumentalist sense). (Conjectures and
Refutations, 235).

For these reasons, the deficiencies discovered by the critics in
Popper’s formal definitions were seen by many as critical, precisely
because the most significant of these related to the levels of
verisimilitude of false theories. In 1974, Miller and
Tichý, working independently of each other, demonstrated that
the conditions specified by Popper in his accounts of both qualitative
and quantitative verisimilitude for comparing the truth- and
falsity-contents of theories can be satisfied only when the theories
are true. In the crucially important case of false theories,
however, Popper’s definitions are formally defective. For while Popper
had believed that verisimilitude intersected positively with his
account of corroboration, in the sense that he viewed an improbable
theory which had withstood critical testing as one the truth-content
of which is great relative to rival theories, while its
falsity-content (if it exists) would be relatively low, Miller and
Tichý proved, on the contrary, that in the case of a false
theory \(t_2\) which has excess content over a rival
theory false \(t_1\) both the truth-content
and the falsity-content of \(t_2\) will exceed
that of \(t_1\). With respect to theories which are
false, therefore, Popper’s conditions for comparing levels of
verisimilitude, whether in quantitative and qualitative terms, can
never be met.

Commentators on Popper, with few exceptions, had initially attached
little importance to his theory of verisimilitude. However, after the
failure of Popper’s definitions in 1974, some critics came to see it
as central to his philosophy of science, and consequentially held that
the whole edifice of the latter had been subverted. For his part,
Popper’s response was two-fold. In the first place, while
acknowledging the deficiencies in his own formal account (“my
main mistake was my failure to see at once that … if the
content of a false statement \( a\) exceeds that of a statement
\(b\), then the truth-content of \(a\) exceeds the
truth-content of \(b\), and the same holds of their
falsity-contents”, Objective Knowledge, 371), Popper
argued that “I do think that we should not conclude from the
failure of my attempts to solve the problem [of defining
verisimilitude] that the problem cannot be solved”
(Objective Knowledge, 372), a point of view which was to
precipitate more than two decades of important technical research in
this field. At another, more fundamental level, he moved the task of
formally defining the concept from centre-stage in his philosophy of
science, by protesting that he had never intended to imply “that
degrees of verisimilitude … can ever be numerically determined,
except in certain limiting cases” (Objective Knowledge,
59), and arguing instead that the chief value of the concept is
heuristic and intuitive, in which the absence of an adequate formal
definition is not an insuperable impediment to its utilisation in the
actual appraisal of theories relativised to problems in which we have
an interest. The thrust of the latter strategy seems to many to
genuinely reflect the significance of the concept of verisimilitude in
Popper’s system, but it has not satisfied all of his critics.

6. Social and Political Thought—The Critique of Historicism and Holism

Given Popper’s personal history and background, it is hardly
surprising that he developed a deep and abiding interest in social and
political philosophy. However, it is worth emphasising that his angle
of approach to these fields is through a consideration of the nature
of the social sciences which seek to describe and explicate them
systematically, particularly history. It is in this context that he
offers an account of the nature of scientific prediction, which in
turn allows him a point of departure for his attack upon
totalitarianism and all its intellectual supports, especially holism
and historicism. In this context holism is to be understood as the
view that human social groupings are greater than the sum of their
members, that such groupings are ‘organic’ entities in
their own right, that they act on their human members and shape their
destinies, and that they are subject to their own independent laws of
development. Historicism, which is closely associated with holism, is
the belief that history develops inexorably and necessarily according
to certain principles or rules towards a determinate end (as for
example in the dialectic of Hegel, which was adopted and implemented
by Marx). The link between holism and historicism is that the holist
believes that individuals are essentially formed by the social
groupings to which they belong, while the historicist—who is
usually also a holist—holds that we can understand such a social
grouping only in terms of the internal principles which determine its
development.

These beliefs lead to what Popper calls ‘The Historicist
Doctrine of the Social Sciences’, the views (a) that the
principal task of the social sciences is to make predictions about the
social and political development of man, and (b) that the task of
politics, once the key predictions have been made, is, in Marx’s
words, to lessen the ‘birth pangs’ of future social and
political developments. Popper thinks that this view of the social
sciences is both theoretically misconceived (in the sense of being
based upon a view of natural science and its methodology which is
totally wrong), and socially dangerous, as it leads inevitably to
totalitarianism and authoritarianism—to centralised governmental
control of the individual and the attempted imposition of large-scale
social planning. Against this Popper strongly advances the view that
any human social grouping is no more (or less) than the sum of its
individual members, that what happens in history is the (largely
unplanned and unforeseeable) result of the actions of such
individuals, and that large scale social planning to an antecedently
conceived blueprint is inherently misconceived—and inevitably
disastrous—precisely because human actions have consequences
which cannot be foreseen. Popper, then, is an historical
indeterminist, insofar as he holds that history does not evolve
in accordance with intrinsic laws or principles, that in the absence
of such laws and principles unconditional prediction in the social
sciences is an impossibility, and that there is no such thing as
historical necessity.

The link between Popper’s theory of knowledge and his social
philosophy is his fallibilism—just as we make theoretical
progress in science by deliberately subjecting our theories to
critical scrutiny, and abandoning those which have been falsified, so
too, Popper holds, the critical spirit can and should be sustained at
the social level. More specifically, the open society can be brought
about only if it is possible for the individual citizen to evaluate
critically the consequences of the implementation of government
policies, which can then be abandoned or modified in the light of such
critical scrutiny—in such a society, the rights of the
individual to criticise administrative policies will be formally
safeguarded and upheld, undesirable policies will be eliminated in a
manner analogous to the elimination of falsified scientific theories,
and differences between people on social policy will be resolved by
critical discussion and argument rather than by force. The open
society as thus conceived of by Popper may be defined as ‘an
association of free individuals respecting each other’s rights within
the framework of mutual protection supplied by the state, and
achieving, through the making of responsible, rational decisions, a
growing measure of humane and enlightened life’ (Levinson, R.B.
In Defense of Plato, 17). As such, Popper holds, it is not a
utopian ideal, but an empirically realised form of social organisation
which, he argues, is in every respect superior to its (real or
potential) totalitarian rivals. But he does not engage in a moral
defence of the ideology of liberalism; rather his strategy is the much
deeper one of showing that totalitarianism is typically based upon
historicist and holist presuppositions, and of demonstrating that
these presuppositions are fundamentally incoherent.

7. Scientific Knowledge, History, and Prediction

At a very general level, Popper argues that historicism and holism
have their origins in what he terms ‘one of the oldest dreams of
mankind—the dream of prophecy, the idea that we can know what
the future has in store for us, and that we can profit from such
knowledge by adjusting our policy to it.’ (Conjectures and
Refutations, 338). This dream was given further impetus, he
speculates, by the emergence of a genuine predictive capability
regarding such events as solar and lunar eclipses at an early stage in
human civilisation, which has of course become increasingly refined
with the development of the natural sciences and their concomitant
technologies. The kind of reasoning which has made, and continues to
make, historicism plausible may, on this account, be reconstructed as
follows: if the application of the laws of the natural sciences can
lead to the successful prediction of such future events as eclipses,
then surely it is reasonable to infer that knowledge of the laws of
history as yielded by a social science or sciences (assuming that such
laws exist) would lead to the successful prediction of such future
social phenomena as revolutions? Why should it be possible to predict
an eclipse, but not a revolution? Why can we not conceive of a social
science which could and would function as the theoretical natural
sciences function, and yield precise unconditional predictions in the
appropriate sphere of application? These are amongst the questions
which Popper seeks to answer, and in doing so, to show that they are
based upon a series of misconceptions about the nature of science, and
about the relationship between scientific laws and scientific
prediction.

His first argument may be summarised as follows: in relation to the
critically important concept of prediction, Popper makes a distinction
between what he terms ‘conditional scientific
predictions’, which have the form ‘If \(X\) takes
place, then \(Y\) will take place’, and
‘unconditional scientific prophecies’, which have the form
‘\(Y\) will take place’. Contrary to popular belief,
it is the former rather than the latter which are typical of the
natural sciences, which means that typically prediction in natural
science is conditional and limited in scope—it takes the form of
hypothetical assertions stating that certain specified changes will
come about if particular specified events antecedently take place.
This is not to deny that ‘unconditional scientific
prophecies’, such as the prediction of eclipses, for example, do
take place in science, and that the theoretical natural sciences make
them possible. However, Popper argues that (a) these unconditional
prophecies are not characteristic of the natural sciences,
and (b) that the mechanism whereby they occur, in the very limited way
in which they do, is not understood by the historicist.

What is the mechanism which makes unconditional scientific prophecies
possible? The answer is that such prophecies can sometimes be derived
from a combination of conditional predictions (themselves derived from
scientific laws) and existential statements specifying that
the conditions in relation to the system being investigated are
fulfilled. Schematically, this can be represented as follows:

\[
[\mathrm{C.P.} + \mathrm{E.S.}]= \mathrm{U.P.}
\]

where \(\mathrm{C.P.} =\) Conditional Prediction; \(\mathrm{E.S.} =\)
Existential Statement; \(\mathrm{U.P.} = \) Unconditional
Prophecy. The most common examples of unconditional scientific
prophecies in science relate to the prediction of such phenomena as
lunar and solar eclipses and comets.

Given, then, that this is the mechanism which generates unconditional
scientific prophecies, Popper makes two related claims about
historicism: (a) That the historicist does not in fact derive his
unconditional scientific prophecies in this manner from conditional
predictions, and (b) the historicist cannot do so because
long-term unconditional scientific prophecies can be derived from
conditional predictions only if they apply to systems which are
well-isolated, stationary, and recurrent (like our solar system). Such
systems are quite rare in nature, and human society is most
emphatically not one of them.

This, then, Popper argues, is the reason why it is a fundamental
mistake for the historicist to take the unconditional scientific
prophecies of eclipses as being typical and characteristic of the
predictions of natural science—in fact such predictions are
possible only because our solar system is a stationary and repetitive
system which is isolated from other such systems by immense expanses
of empty space. The solar system aside, there are very few such
systems around for scientific investigation—most of the others
are confined to the field of biology, where unconditional prophecies
about the life-cycles of organisms are made possible by the existence
of precisely the same factors. Thus one of the fallacies committed by
the historicist is to take the (relatively rare) instances of
unconditional prophecies in the natural science as constituting the
essence of what scientific prediction is, to fail to see that such
prophecies apply only to systems which are isolated, stationary, and
repetitive, and to seek to apply the method of scientific prophecy to
human society and human history. The latter, of course, is
not an isolated system (in fact it’s not a system at all), it
is constantly changing, and it continually undergoes rapid,
non-repetitive development. In the most fundamental sense possible,
every event in human history is discrete, novel, quite unique, and
ontologically distinct from every other historical event. For this
reason, it is impossible in principle that unconditional scientific
prophecies could be made in relation to human history—the idea
that the successful unconditional prediction of eclipses provides us
with reasonable grounds for the hope of successful unconditional
prediction regarding the evolution of human history turns out to be
based upon a gross misconception, and is quite false. As Popper
himself concludes, “The fact that we predict eclipses does not,
therefore, provide a valid reason for expecting that we can predict
revolutions.” (Conjectures and Refutations, 340).

8. Immutable Laws and Contingent Trends

This argument is one of the strongest that has ever been brought
against historicism, cutting, as it does, right to the heart of one of
its main theoretical presuppositions. However, it is not Popper’s only
argument against it. An additional mistake which he detects in
historicism is the failure of the historicist to distinguish between
scientific laws and trends, which is also frequently
accompanied by a simple logical fallacy. The fallacy is that of
inferring from the fact that our understanding of any (past)
historical event—such as, for example, the French
Revolution—is in direct proportion to our knowledge of the
antecedent conditions which led to that event, that knowledge of all
the antecedent conditions of some future event is possible, and that
such knowledge would make that future event precisely predictable. For
the truth is that the number of factors which predate and lead to the
occurrence of any event, past, present, or future, is indefinitely
large, and therefore knowledge of all of these factors is impossible,
even in principle. What gives rise to the fallacy is the manner in
which the historian (necessarily) selectively isolates a finite number
of the antecedent conditions of some past event as being of particular
importance, which are then somewhat misleadingly termed ‘the
causes’ of that event, when in fact what this means is that they
are the specific conditions which a particular historian or group of
historians take to be more relevant than any other of the
indefinitely large number of such conditions (for this reason, most
historical debates range over the question as to whether the
conditions thus specified are the right ones). While this
kind of selectivity may be justifiable in relation to the treatment of
any past event, it has no basis whatsoever in relation to the
future—if we now select, as Marx did, the ‘relevant’
antecedent conditions for some future event, the likelihood is that we
will select wrongly.

The historicist’s failure to distinguish between scientific laws and
trends is equally destructive of his cause. This failure makes him
think it possible to explain change by discovering trends running
through past history, and to anticipate and predict future occurrences
on the basis of such observations. Here Popper points out that there
is a critical difference between a trend and a scientific law, the
failure to observe which is fatal. For a scientific law is universal
in form, while a trend can be expressed only as a singular existential
statement. This logical difference is crucial because unconditional
predictions, as we have already seen, can be based only upon
conditional ones, which themselves must be derived from scientific
laws. Neither conditional nor unconditional predictions can be based
upon trends, because these may change or be reversed with a change in
the conditions which gave rise to them in the first instance. As
Popper puts it, there can be no doubt that “the habit of
confusing trends with laws, together with the intuitive observation of
trends such as technical progress, inspired the central doctrines of
… historicism.” (The Poverty of Historicism,
116). Popper does not, of course, dispute the existence of trends, nor
does he deny that the observation of trends can be of practical
utility value—but the essential point is that a trend is
something which itself ultimately stands in need of
scientific explanation, and it cannot therefore function as the frame
of reference in terms of which anything else can be scientifically
explained or predicted.

A point which connects with this has to do with the role which the
evolution of human knowledge has played in the historical development
of human society. It is incontestable that, as Marx himself observed,
there has been a causal link between the two, in the sense that
advances in scientific and technological knowledge have given rise to
widespread global changes in patterns of human social organisation and
social interaction, which in turn have led to social structures (e.g.
educational systems) which further growth in human knowledge. In
short, the evolution of human history has been strongly influenced by
the growth of human knowledge, and it is extremely likely
that this will continue to be the case—all the empirical
evidence suggests that the link between the two is progressively
consolidating. However, this gives rise to further problems for the
historicist. In the first place, the statement that ‘if there is
such a thing as growing human knowledge, then we cannot anticipate
today what we shall know only tomorrow’ is, Popper holds,
intuitively highly plausible. Moreover, he argues, it is logically
demonstrable by a consideration of the implications of the fact that
no scientific predictor, human or otherwise, can possibly predict, by
scientific methods, its own future results. From this it follows, he
holds, that ‘no society can predict, scientifically, its own
future states of knowledge’. (The Poverty of
Historicism, vii). Thus, while the future evolution of human
history is extremely likely to be influenced by new developments in
human knowledge, as it always has in the past, we cannot now
scientifically determine what such knowledge will be. From this it
follows that if the future holds any new discoveries or any new
developments in the growth of our knowledge (and given the fallible
nature of the latter, it is inconceivable that it does not), then it
is impossible for us to predict them now, and it is therefore
impossible for us to predict the future development of human history
now, given that the latter will, at least in part, be determined by
the future growth of our knowledge. Thus once again historicism
collapses—the dream of a theoretical, predictive science of
history is unrealisable, because it is an impossible dream.

Popper’s arguments against holism, and in particular his arguments
against the propriety of large-scale planning of social structures,
are interconnected with his demonstration of the logical shortcomings
of the presuppositions of historicism. Popper points out that such
planning (which actually took place, of course, in the USSR, in China,
and in Cambodia, for example, under totalitarian regimes which
accepted forms of historicism and holism) is necessarily structured in
the light of the predictions which have been made about future history
on the basis of the so-called ‘laws’ which historicists
such as Marx and Mao claimed to have discovered in relation to human
history. Accordingly, recognition that there are no such laws, and
that unconditional predictions about future history are based, at
best, upon nothing more substantial than the observation of contingent
trends, shows that, from a purely theoretical as well as a practical
point of view, large-scale social planning is indeed a recipe for
disaster. In summary, unconditional large-scale planning for the
future is theoretically as well as practically misguided, because,
again, part of what we are planning for is our future knowledge, and
our future knowledge is not something which we can in principle now
possess—we cannot adequately plan for unexpected advances in our
future knowledge, or for the effects which such advances will have
upon society as a whole. The acceptance of historical indeterminism,
then, as the only philosophy of history which is commensurate with a
proper understanding of the nature of scientific knowledge, fatally
undermines both historicism and holism.

Popper’s critique of both historicism and holism is balanced, on the
positive side, by his affirmation of the ideals of individualism and
market economics and his strong defence of the open society—the
view, again, that a society is equivalent to the sum of its members,
that the actions of the members of society serve to fashion and to
shape it, and that the social consequences of intentional actions are
very often, and very largely, unintentional. This part of his social
philosophy was influenced by the economist Friedrich Hayek, who worked
with him at the London School of Economics and who was a life-long
friend. Popper advocated what he (rather unfortunately) terms
‘piecemeal social engineering’ as the central mechanism
for social planning—for in utilising this mechanism intentional
actions are directed to the achievement of one specific goal at a
time, which makes it possible to monitor the situation to determine
whether adverse unintended effects of intentional actions occur, in
order to correct and readjust when this proves necessary. This, of
course, parallels precisely the critical testing of theories in
scientific investigation. This approach to social planning (which is
explicitly based upon the premise that we do not, because we cannot,
know what the future will be like) encourages attempts to put right
what is problematic in society—generally-acknowledged social
ills—rather than attempts to impose some preconceived idea of
the ‘good’ upon society as a whole. For this reason, in a
genuinely open society piecemeal social engineering goes hand-in-hand
for Popper with negative utilitarianism (the attempt to
minimise the amount of misery, rather than, as with positive
utilitarianism, the attempt to maximise the amount of happiness). The
state, he holds, should concern itself with the task of progressively
formulating and implementing policies designed to deal with the social
problems which actually confront it, with the goal of eliminating
human misery and suffering to the highest possible degree. The
positive task of increasing social and personal happiness, by
contrast, can and should be left to individual citizens (who
may, of course, act collectively to this end), who, unlike the state,
have at least a chance of achieving this goal, but who in a free
society are rarely in a position to systematically subvert the rights
of others in the pursuit of idealised objectives. Thus in the final
analysis for Popper the activity of problem-solving is as definitive
of our humanity at the level of social and political organisation as
it is at the level of science, and it is this key insight which
unifies and integrates the broad spectrum of his thought.

9. Critical Evaluation

While it cannot be said that Popper was a modest man, he took
criticism of his theories very seriously, and spent much of his time
in his later years trying to show that such criticisms were either
based upon misunderstandings, or that his theories could, without loss
of integrity, be made compatible with new and important insights. The
following is a summary of some of the main criticisms which he has had
to address. (For Popper’s responses to critical commentary, see his
‘Replies to My Critics’, in P.A. Schilpp (ed.), The
Philosophy of Karl Popper, Volume 2, and his Realism and the
Aim of Science, edited by W.W. Bartley III.)

1. Popper professes to be anti-conventionalist, and his commitment to
the correspondence theory of truth places him firmly within the
realist’s camp. Yet, following Kant, he strongly repudiates the
positivist/empiricist view that basic statements (i.e., present-tense
observation statements about sense-data) are infallible, and argues
convincingly that such basic statements are not mere
‘reports’ of passively registered sensations. Rather they
are descriptions of what is observed as interpreted by the observer
with reference to a determinate theoretical framework. This is why
Popper repeatedly emphasises that basic statements are not infallible,
and it indicates what he means when he says that they are
‘theory laden’—perception itself is an active
process, in which the mind assimilates data by reference to an assumed
theoretical backdrop. He accordingly asserts that basic statements
themselves are open-ended hypotheses: they have a certain causal
relationship with experience, but they are not determined by
experience, and they cannot be verified or confirmed by experience.
However, this poses a difficulty regarding the consistency of Popper’s
theory: if a theory \(X\) is to be genuinely testable (and so
scientific) it must be possible to determine whether or not the basic
propositions which would, if true, falsify it, are actually
true or false (i.e., whether its potential falsifiers are actual
falsifiers). But how can this be known, if such basic statements
cannot be verified by experience? Popper’s answer is that ‘basic
statements are not justifiable by our immediate experiences, but are
… accepted by an act, a free decision’. (Logic of
Scientific Discovery, 109). However, and notwithstanding Popper’s
claims to the contrary, this itself seems to be a refined form of
conventionalism—it implies that it is almost entirely an
arbitrary matter whether it is accepted that a potential falsifier is
an actual one, and consequently that the falsification of a theory is
itself the function of a ‘free’ and arbitrary act. It also
seems very difficult to reconcile this with Popper’s view that science
progressively moves closer to the truth, conceived of in terms of the
correspondence theory, for this kind of conventionalism is inimical to
this (classical) conception of truth.

2. As Lakatos has pointed out, Popper’s theory of demarcation hinges
quite fundamentally on the assumption that there are such things as
critical tests, which either falsify a theory, or give it a strong
measure of corroboration. Popper himself is fond of citing, as an
example of such a critical test, the resolution, by Adams and
Leverrier, of the problem which the anomalous orbit of Uranus posed
for nineteenth century astronomers. Both men independently came to the
conclusion that, assuming Newtonian mechanics to be precisely correct,
the observed divergence in the elliptical orbit of Uranus could be
explained if the existence of a seventh, as yet unobserved outer
planet was posited. Further, they were able, again within the
framework of Newtonian mechanics, to calculate the precise position of
the ‘new’ planet. Thus when subsequent research by Galle
at the Berlin observatory revealed that such a planet (Neptune) did in
fact exist, and was situated precisely where Adams and Leverrier had
calculated, this was hailed as by all and sundry as a magnificent
triumph for Newtonian physics: in Popperian terms, Newton’s theory had
been subjected to a critical test, and had passed with flying colours.
Popper himself refers to this strong corroboration of Newtonian
physics as ‘the most startling and convincing success of any
human intellectual achievement’. Yet Lakatos flatly denies that
there are critical tests, in the Popperian sense, in science, and
argues the point convincingly by turning the above example of an
alleged critical test on its head. What, he asks, would have happened
if Galle had not found the planet Neptune? Would Newtonian
physics have been abandoned, or would Newton’s theory have been
falsified? The answer is clearly not, for Galle’s failure could have
been attributed to any number of causes other than the falsity of
Newtonian physics (e.g., the interference of the earth’s atmosphere
with the telescope, the existence of an asteroid belt which hides the
new planet from the earth, etc). The point here is that the
‘falsification/corroboration’ disjunction offered by
Popper is far too logically neat: non-corroboration is not
necessarily falsification, and falsification of a high-level
scientific theory is never brought about by an isolated observation or
set of observations. Such theories are, it is now generally accepted,
highly resistant to falsification. They are falsified, if at all,
Lakatos argues, not by Popperian critical tests, but rather within the
elaborate context of the research programmes associated with them
gradually grinding to a halt, with the result that an ever-widening
gap opens up between the facts to be explained, and the research
programmes themselves (Lakatos 1978, passim). Popper’s distinction
between the logic of falsifiability and its applied methodology does
not in the end do full justice to the fact that all high-level
theories grow and live despite the existence of anomalies (i.e.,
events/phenomena which are incompatible with the theories). The
existence of such anomalies is not usually taken by the working
scientist as an indication that the theory in question is false; on
the contrary, he will usually, and necessarily, assume that the
auxiliary hypotheses which are associated with the theory can be
modified to incorporate, and explain, existing anomalies.

3. Scientific laws are expressed by universal statements (i.e., they
take the logical form ‘All \(A\)s are \(X\)’, or
some equivalent) which are therefore concealed conditionals—they
have to be understood as hypothetical statements asserting what would
be the case under certain ideal conditions. In themselves they are not
existential in nature. Thus ‘All \(A\)s are
\(X\)’ means ‘If anything is an \(A\), then it
is \(X\)’. Since scientific laws are non-existential in
nature, they logically cannot imply any basic statements, since the
latter are explicitly existential. The question arises, then, as to
how any basic statement can falsify a scientific law, given that basic
statements are not deducible from scientific laws in themselves?
Popper answers that scientific laws are always taken in
conjunction with statements outlining the ‘initial
conditions’ of the system under investigation; these latter,
which are singular existential statements, do, when combined with the
scientific law, yield hard and fast implications. Thus, the law
‘All \(A\)s are \(X\)’, together with the
initial condition statement ‘There is an \(A\) at
\(Y\)’, yields the implication ‘The \(A\) at
\(Y\) is \(X\)’, which, if false, falsifies the
original law.

This reply is adequate only if it is true, as Popper assumes, that
singular existential statements will always do the work of
bridging the gap between a universal theory and a prediction. Hilary
Putnam in particular has argued that this assumption is false, in that
in some cases at least the statements required to bridge this gap
(which he calls ‘auxiliary hypotheses’) are general rather
than particular, and consequently that when the prediction turns out
to be false we have no way of knowing whether this is due to the
falsity of the scientific law or the falsity of the auxiliary
hypotheses. The working scientist, Putnam argues, always initially
assumes that it is the latter, which shows not only that scientific
laws are, contra Popper, highly resistant to falsification,
but also why they are so highly resistant to falsification.

Popper’s final position is that he acknowledges that it is impossible
to discriminate science from non-science on the basis of the
falsifiability of the scientific statements alone; he
recognizes that scientific theories are predictive, and consequently
prohibitive, only when taken in conjunction with auxiliary
hypotheses, and he also recognizes that readjustment or modification
of the latter is an integral part of scientific practice. Hence his
final concern is to outline conditions which indicate when such
modification is genuinely scientific, and when it is merely ad
hoc. This is itself clearly a major alteration in his position,
and arguably represents a substantial retraction on his part: Marxism
can no longer be dismissed as ‘unscientific’ simply
because its advocates preserved the theory from falsification by
modifying it (for in general terms, such a procedure, it now
transpires, is perfectly respectable scientific practice). It is now
condemned as unscientific by Popper because the only
rationale for the modifications which were made to the
original theory was to ensure that it evaded falsification, and so
such modifications were ad hoc, rather than scientific. This
contention—though not at all implausible—has, to hostile
eyes, a somewhat contrived air about it, and is unlikely to worry the
convinced Marxist. On the other hand, the shift in Popper’s own basic
position is taken by some critics as an indicator that
falsificationism, for all its apparent merits, fares no better in the
final analysis than verificationism.

Sturm, T. 2012. ‘Bühler and Popper: Kantian Therapies
for the Crisis in Psychology’, Studies in History and
Philosophy of Science Part C Studies in History and Philosophy of
Biological and Biomedical Sciences, 43(2): 462–72.

Tichý, P., 1974, ‘On Popper’s Definitions of
Verisimilitude’, The British Journal for the Philosophy of
Science, 25: 155–160